How to update 12.4.2 version

There are no updates for 12.4.2.

A iPhone 6 can't install iOS 13.

Download it from your iPhone.

No one here can send it to you.

You can't. 12.4.2 was only released for devices incompatible of supporting iOS 13.

You would need to buy a new iPhone if you want iOS 13. iPhone 6 cannot run it.